Controlling sulfide scavenger content of drilling fluid

ABSTRACT

The concentration of unspent zinc-based hydrogen sulfide scavenger in an aqueous drilling fluid is controlled by selectively extracting the unspent scavenger in a solvent, such as glacial acetic acid, measuring the concentration of dissolved zinc, for example, with an X-ray fluorescence spectrograph, and utilizing the results of the measurements to proportion the extent of changes to be made in the concentration of the scavenger.

RELATED PATENT APPLICATION

This is a continuation-in-part of patent application Ser. No. 609,611filed May 14, 1984, now abandoned, the disclosures of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to determining and utilizing the amount ofunspent zinc-based sulfide scavenger which is present in a water-baseddrilling fluid for adjusting the scavenging capability of the drillingfluid to the extent desired during the drilling of a well. Moreparticularly, the invention relates to a relatively quick and accurateprocedure which can be used in field locations.

A state of the art paper entitled "Chemical Scavengers for Sulfides inWater-Based Drilling Fluids" by R. L. Garrett, R. K. Clark, L. L. Carneyand C. K. Grantham, Sr. in Journal of Petroleum Technology, June 1979,page 787, discusses the chemistry of commercial scavengers forwater-based drilling fluids, the parameters that affect the reliabilityof such materials and the problems affecting scavenger use. On page 796,the authors point out that "From this state of the art review one cansee that we believe more research is needed to develop scavengers andtests for scavenger content in muds that match more closely thequalities of an ideal scavenger."

U.S. Pat. No. 3,706,532 describes a method for determining zincconcentrations in aqueous mediums. A sample mediums is acidified,treated with a buffered complexing agent to complex aluminum or ironions, treated with an organo sulfur compound to complex copper ions,then analyzed by adding an indicator the color intensity of which iscalibrated with respect to known concentrations of zinc. U.S. Pat. No.3,928,211 describes a class of zinc carbonate, basic zinc carbonate andzinc hydroxide compounds effective for sulfide scavenging. U.S. Pat. No.4,252,655 describes the removal or inactivation of hydrogen sulfidecontamination by adding at least one organic zinc chelate.

SUMMARY OF THE INVENTION

The present invention relates to improving a process for drilling a wellwith an aqueous drilling fluid containing a zinc-based scavenger ofsulfide ions. A determination is made of the amount of unspentzinc-containing sulfide-scavenging material present in the drillingfluid. A measured volume of the drilling fluid is mixed with asignificantly larger number of volumes (such as about 6 to 10) of aselective solvent for dissolving zinc ions and establishing within theresulting mixture a pH (such as a pH of from about 4 to 6) at whichsubstantially all of the zinc in the drilling fluid, except for thatcombined into zinc sulfide molecules, becomes dissolved in the liquidphase of the mixture. A portion of the resulting liquid solution isseparated from the solid components of the drilling fluid and the amountof zinc contained in the solids-free liquid is determined in order todetermine the amount of unspent zinc-containing sulfide scavenger in thedrilling fluid. The amount of scavenger in the mud is then adjusted tothe extent required to provide a capability of precipitating a selectedamount of sulfide ions without involving a solids content capable ofimpairing the drilling fluid rheology.

In a preferred embodiment of the invention the drilling fluid sample ismixed with about 4 to 10 times its volume of glacial acetic acid, or aselective solvent which is substantially equivalent to glacial aceticacid with respect to selectively dissolving zinc ions which have notcombined with sulfide ions. The concentration of zinc in the resultingsolution is preferably measured with a portable X-ray fluorescencespectrographic unit which is, or is substantially equivalent to, aPortaspec Model 2501 portable X-ray spectrograph (available fromPitchford Scientific Instruments Division of the Hankison Corporation).

In a preferred procedure, for example, in situations in which theproportions found of unspent zinc based scavenger are relatively low, anaugmentive test for total zinc (including that combined into zincsulfide molecules) can be performed by (a) an X-ray fluorescencemeasurement, or equivalent measurement, of the zinc in the unleacheddrilling fluid or, (b) using as the solvent for dissolving zinc from thedrilling fluid a strong acid, such as hydrochloric acid, as a solvent,for combined and non-combined zinc, prior to measuring the concentrationof the zinc solution. Such an acid preferably has a normality of fromabout 1 to 3. The difference between the prior and augmentive tests willindicate whether the scavenger concentration was reduced by dilution ofthe drilling fluid or by combination with sulfide.

DESCRIPTION OF THE INVENTION

Applications have discovered that possible needs for changing theconcentration of zinc base scavenger in a drilling fluid can beaccurately monitored at the well site so the corrections in the rate ofscavenger addition can be properly initiated. This can be effected byutilization of the present process. This process enables the drillingfluid to be sampled at a selected frequency with the results ofdeterminations of the concentrations of unspent scavenger promptlyavailable to the mud engineers. For example, within about 30 minutes orso, based on such information, increases or decreases can be made in therate of scavenger addition and for additions of scavenger-free fluid tothe extent needed to quickly change that concentration to either avoidan impairment of the drilling fluid rheology or to quickly scavenge asudden encounter or sulfide.

Experiments were conducted using samples of an aqueous drilling fluidtypical of that used in drilling operations. Quadruplicate examples wereperformed on samples of that mud spiked with proportions of 1 lb. perbarrel (ppb) of Sulf-X (a zinc based sulfide scavenger available fromImco Services, a Halliburton Company). The tests employed the followingprocedures, which are preferred procedures for use in the presentinvention.

SAMPLE PREPARATION

1. Measure 10 ml of stirred mud into a 10 ml graduated cylinder using apipet with the end of the pipet cut off to minimize any particle sizeexclusion.

2. Transfer the measured mud sample to a 150 ml beaker.

3. Add 60 ml of glacial acetic acid to the mud sample.

4. Heat at about 110° C. with frequency stirring for 10-15 minutes.

5. Allow the solution to cool sufficiently to prevent damage to aplastic centrifuge tube.

6. Place a portion of the mud-acetic acid mixture into a plastic 50 mlcentrifuge tube.

7. Centrifuge so that all the mud is firmly packed at the bottom of thecentrifuge tube.

8. Accurately pipet 10 ml of the centrifuge solution into a ChemplexX-ray fluorescence counting vial using a 5 ml Finnpipette.

9. Cover the counting vial with polypropylene film, brace the film ontothe vial with a small collar, and fix the film onto the vial with alarge collar.

INSTRUMENTAL MEASUREMENTS BY X-RAY FLUORESCENCE

1. Position the element selector to Zn using the sidearm lever.

2. Open the sample compartment door.

3. Plug into a 110V outlet and engage "Power" button. Wait for the"ready" light and let warm 10 minutes.

4. Place sample counting vials in the spring-loaded mount. Insert themount into the sample chamber with the rounded edge of the stainlessmount facing inward. [Note: Make sure no droplets are present on theundersurface of the polypropylene film. These droplets will cause anerrant increase in count rate.]

5. Close the sample chamber door and check to see if the "X-rays on"indicator is illuminated. If it is not illuminated, the stainlessplanchet holder should be reinserted in the other direction.

6. With "X-rays on", adjust the current to read 0.5 milliamps.

7. Set the counting scaler on the front panel to 60 seconds.

8. Engage count pushbutton and record the final gross X-ray intensitycounts on the digital readout.

9. Obtain gross X-ray counts for the glacial acetic acid blank and acalibration standard prepared y the dissolution of ZnO in glacial aceticacid.

10. To leave instrument in standby position, open the sample compartmentdoor.

11. For longer periods of inactivity, turn down the current, turn offmain power and unplug.

CALCULATIONS (based on the following conditions)

10 ml mud, 60 ml acetic acid, 10 ml aliquots in counting vial.

Calculations are not valid for variations from these amounts.

1. Determine net counts for samples and ZnO calibration standard bysubtracting the glacial acetic acid blank counts.

2. Determine the mg of Zn in 10 ml mud sample by the following ratio:##EQU1## 3. Determine ppb (pounds per barrel) Zn by multiplying the mgZn in the 10 ml mud sample by 0.035. The factor 0.035 is derived fromthe following conversion: ##EQU2## 4. Determine ppb Sulf-X bymultiplying ppb Zn by 1.67 [Sulf-X contains 60.0% Zn].

TEST RESULTS

The tests indicated the following:

Sulf-X was experimentally determined to be present at 0.97±0.09 ppb.These results indicate the accuracy and precision of the method to bewithin the 10 percent relative objective.

Additional experiments were designed to simulate situations where thescavenger containing mud had been totally exhausted by hydrogen sulfideintrusion. This was accomplished by spiking mud with 1 ppb zinc sulfidewhich is the product from the reaction of the zinc scavenger withsulfide. Duplicate analyses yielded unspent scavenger concentrations of0.02±0.01 ppb indicating that the acetic acid leach is effective atdifferentiating spent and unspent zinc scavenger.

In a third experiment, unspiked mud was found to have 0.03 ppb unspentzinc scavenger which indicates that potential interferents inherent tothe mud are virtually non-existent.

In general, the selective solvent for zinc ions can comprisesubstantially any buffered liquid having a composition and concentrationcapable of providing a pH of about 4 to 6 when one part by volume of adrilling fluid having a pH in the range of from about 9 to 12 is mixedwith about 4 to 10 parts by volume of said liquid. Examples of suitableselective solvent solutions include: glacial acetic acid, 10% formicacid, and 0.0001 M hydrochloric acid.

In general, the concentration of zinc which becomes dissolved in theselective solvent can be measured by substantially any suitably accurateprocedure. Procedures capable of being conducted in field locations arepreferred. An example of such a procedure is described in "ColorimetricDeterminations of Elements" by G. Charlot, Elsevier Publishing Company,1964.

Suitable Compositions and Procedures for Use in the Invention

The present invention is applicable to substantially any process fordrilling the borehole of a well with an aqueous drilling fluid in alocation in which the wellbore may encounter water soluble sulfide ionssuch as those in hydrogen sulfide or salts containing HS⁻ or S.sup.∛.Such acids and salts commonly coexist in a subterraneansulfide-containing water system.

In a preferred embodiment of the invention, the above described analysesare conducted at the drilling site with a frequency which increases withthe likelihood of the borehole encountering sulfide ions and/orincreases in the extent by which the zinc-based scavenger is found tohave been depleted by round trips of the circulating drilling fluid. Thezinc-based sulfide scavengers are generally available as solids and canbe added as dry solids through a hopper for mixing solids with thecirculating drilling fluid. But, in a preferred procedure, thescavengers are preferably added in the form of slurries in aqueousliquids. In addition, as known in the art, a lignosulfonate treatment ofthe drilling fluid can be utilized for controlling any undesirablezinc-induced flocculation of mud components.

In general, the most commonly used zinc based sulfide scavenger is abasic zinc carbonate. It is a manufactured compound having a formulaaveraging about 3Zn(OH)₂. 2Zn CO₃. As known in the art, where desirableto minimize any adverse effects of zinc ions, those ions can be looselybonded with organic compounds into the form of metal chelates.Commercially available zinc chelates are based on aliphatic amino acidsor their salts. Such chelated zinc ions tend to avoid being captured onclay surfaces in a manner causing flocculation while still beingavailable for precipitating sulfide ions.

What is claimed is:
 1. In a process for drilling a well with a waterbased drilling fluid containing zinc based scavengers for controllingencountered sulfide ions, in which process analyses are made formonitoring and controlling the amount of scavenger in the drillingfluid, an improved process for controlling the adequacy of thescavenging capability, comprising:mixing one part by volume of a sampleof circulating drilling fluid with about 4 to 10 parts by volume of aselective solvent for zinc ions, with said solvent .[.containing a kindand amount of buffering material.]. providing a pH of about 4 to 6, andbeing capable of dissolving substantially all of the zinc present in thedrilling fluid except for zinc ions combined into .[.precipitatedmolecules of.]. zinc sulfides; separating the resulting solution fromundissolved solids; determining the amount of zinc in the solids-freesolution in order to determine the proportion of the unspent zinc basedsulfide scavenger in the circulating drilling fluid; and adjusting theproportion of the scavenger in the drilling fluid to the excent requiredto provide enough for scavenging a selected amount of encounteredsulfide ions without providing enough to impair the rheology of thedrilling fluid.
 2. The process of claim 1 in which the amount of zinc inthe solution is determined by measuring the amount of X-ray fluorescenceexhibited by the solution.
 3. The process of claim 1 in which theselective solvent for zinc is glacial acetic acid.
 4. The process ofclaim 1 in which the selective solvent for zinc is glacial acetic acid,the mixture of drilling fluid and selective solvent contains about 1part by volume of the drilling fluid per 4 to 10 parts by volume of thesolvent and the amount of zinc which becomes dissolved in that solutionis determined by a measurement of X-ray fluorescence.
 5. The process ofclaim 1 in which an additional portion of the same drilling fluid issimilarly mixed with solvent consisting of an aqueous strong acidsolution capable of dissolving substantially all combined andnon-combined zinc in the drilling fluid, with the amount of zinc in thesolution being similarly measured to determine the decrease in scavengerdue to drilling fluid dilution and reaction of scavenger with sulfides.6. The process of claim 5 in which the strong acid is hydrochloric acidhaving a normality of about 1 to
 3. 7. The process of claim 1 in whichthe amount of zinc dissolved in the selective solvent for zinc ions isdetermined by colorimetric analysis.
 8. The process of claim 1 in whichthe zinc-based sulfide scavenger is a basic zinc carbonate.
 9. Theprocess of claim 1 in which the zinc-based sulfide scavenger is a watersoluble zinc chelate.
 10. The process of claim 1 in which determinationsof the proportion of unspent zinc-based scavenger are (a) initiated at afrequency which provides a rate of testing that varies with variationsin the likelihood of the borehole encountering changed sulfide ionconcentrations which change the extent to which the amount of unspentscavenger is different between the tests, (b) are conducted at the drillsite and, (c) to the extend required are utilized to proportion changesin the concentration of scavenger in the drilling fluid for keeping thatconcentration high enough to scavenge the sulfide being encounteredwithout becoming high enough to impair the drilling fluid rheology. 11.In a process for drilling a well with water based drilling fluidcontaining zinc-based scavengers for controlling encountered sulfideions in which process analyses are made for monitoring and controllingthe amount of scavenger in the drilling fluid, an improved process forcontrolling the adequancy of the scavenging capability,comprising:mixing one part by volume of a sample of circulating drillingfluid with about 4 to 10 parts by volume of a selected solvent for zincions with said solvent containing, glacial acetic acid and in an amountproviding a pH of about 4 to 6, said solvent being capable ofselectively dissolving zinc ions which have not combined with sulfideions and being capable of dissolving substantially all of the zincpresent in the drilling fluid except for zinc ions combined into.[.precipitated.]. molecules of zinc sulfides; separating the resultingsolution from undissolved solids; determining the amount of zinc in thesolids free solution by measuring the concentration of said zinc ions byX-ray fluorescence to determine the concentration of unspent zinc-basedsulfide scavenger in the circulating drilling fluid; and adjusting theproportion of said scavenger to the extent required to provide enoughfor scavenging a selected amount of encountered sulfide ions withoutproviding enough to impair the rheology of the drilling fluid.